Unexpectedly, many chondrites, of every type, contain secondary minerals that must have formed as the primary mineral reacted with liquid water. The reactions took place very soon after the formation of the chondrules and often during accretion of the incorporating bodies (Doyle et al. 2015). In some cases the minerals hydrated even before accretion. Water molecules suspended in space were wetting the grain surfaces and making them stickier, accelerating the process by which the grains coalesced. Carbonaceous chondrites tend to be particularly water-rich, possibly because they formed beyond the ‘snow line’ within which the infant Sun’s heat would have prevented the volatile from condensing.
Hydrated minerals have also been detected in comets and asteroids, both remotely and in samples recovered by probes. In the case of the asteroid Ryugu the hydration was dated to within the first 1.8 Ma of the solar system’s history (McCain et al. 2023). Ceres, the only asteroid large enough to be rounded by its own gravity, is in fact more like a comet, with 50% of its volume estimated to be ice (27% by mass). Cryovolcanoes occasionally erupt salty water from its interior. Comets, of course, contain copious amounts of frozen water. However, the evidence of liquid water shatters the paradigm that comets never get warm enough to melt their icy interiors (Berger et al. 2011).
Interplanetary space was wet. Evidence for this doesn’t just come from asteroids and comets. All the terrestrial planets show signs of having once been drenched by water – even Mercury. One of the most astonishing findings of the Messenger mission to Mercury was that under the walls of high-latitude craters untouched by solar radiation water abounds. Deposits 50 m thick are thought to lie beneath the rims.
No less surprising is the evidence from Venus, the hottest planet in the Solar System. Although Venus today has a dry surface and is shrouded under clouds of carbon dioxide and sulphuric acid, the high ratio of deuterium to hydrogen in its atmosphere suggests that it once hosted a substantial ocean, subsequently evaporated or blasted away. Deuterium is an isotope of hydrogen, heavier than the ordinary form because the nucleus includes a neutron. It combines with oxygen to produce a heavy form of water. The idea is that when ultraviolet radiation from the Sun split the evaporated water into hydrogen, deuterium and oxygen, the lightest gas, hydrogen, and most of the deuterium escaped into space, whereas some of the deuterium remained in the atmosphere. The heavier oxygen oxidised the crust.
Until recently, the Moon was believed to be devoid of water. Then in October 2009 it was announced that the LCROSS mission (Lunar Crater Observation and Sensing Satellite) had discovered significant quantities of water after crashing part of the spacecraft into a crater close to the permanently shadowed south pole. Five months after that, it was announced that millions of tons of ice were hidden deep within craters around the north pole. The polar ice had to be very ancient. However, because of the difficulty of understanding where it might have come from, scientists continued to doubt that water was present. In August 2018 a paper analysing data gathered by India’s Chandrayaan-1 probe ten years earlier gave the first definitive proof of ice on the surface. The presence of water can be doubted no longer. Analysis of basalts recovered by China’s Chang’e-5 mission indicates that the interior is dry. The most parsimonious overall explanation is therefore that the water originated as droplets settling on the surface from outer space. Some of the water is likely to be a product of the solar wind.
That the mantle is dry is problematic because water lowers a rock’s melting point and if evidence of significant water had been found it might have explained why, as late as 2 billion years ago, the Moon was capable of producing the vast lava plain known as Oceanus Procellarum (Mallapaty 2022). For the moment, assuming that radioactive decay rates were the same as now, the problem remains unsolved.
Oceans of water cover most of the Earth’s surface, to an average depth of almost 4 km. According to the nebula hypothesis, Earth should not have had oceans to start with, since it lies within the snow line. The only ocean would have been a global ‘magma ocean’, driving off whatever water might have been delivered by early-forming comets. Yet water has been abundant on or in the Earth from as far back as datable minerals can take us, in geological time as early as 4.4 Ga ago. At the beginning of the Archaean 4.0 Ga ago, the entire planet is thought to have been under water.
Mars’s early history is also puzzling. Its surface today is cold and dry, yet evidence of former water turns up wherever one looks. The ancient depression in its northern hemisphere once contained an ocean more than 400 metres deep, covering a third of its surface. Formerly water-conducting deltas and valley networks fringe the basin. Within the basin can still be discerned the outlines of smaller craters whose walls were eroded by the ocean and whose floors received thick sheets of sediment. In other regions, craters are surrounded by splashes of ejected matter, showing that the ground possessed, or was shock-heated to, a mud-like consistency. The surface was wet when asteroids bombarded the planet. For many years, clouds continued to rain on the lowlands, repeating their cycles of evaporation, re-precipitation and runoff as the water gradually seeped into the ground.
Approximately 1 bar of CO2 atmosphere is needed to sustain a climate warm enough to maintain liquid water on the surface (Palumbo et al. 2018). While there is no reason why Mars would have been created with such an atmosphere, high levels of CO2 might have built up through high rates of volcanic outgassing. Today the planet is colder, has only a 7 mbar atmosphere (nearly all CO2), and what remains of the water is locked up as ice beneath the surface and in large permanent caps around the poles.
Jupiter mostly consists of hydrogen and helium, the helium being a product of nuclear fusion in the core when the speed of light was much faster than now. Its atmosphere also contains appreciable amounts of oxygen, forming clouds of H2O. At the equator, water makes up about 0.25% of the molecules in Jupiter’s atmosphere. As with the other giant planets, the atmosphere also contains substantial amount of ammonia (NH3) and methane.
Europa, one of the four moons that Galileo detected with his telescope, has a rocky interior with an icy shell; below the ice, an ocean of salty water is inferred. Ganymede, its largest moon and the largest in the solar system, is 50% water. Like Vesta, its rocky interior is differentiated, with a metallic core. Callisto’s low density suggests that it too is 50% water. Io, the innermost and densest of the four moons, is partly molten and has almost no water.
Saturn is best known for its rings, which consist almost entirely of ice particles. One of the rings is fed by vapour ejected from the moon Enceladus, which hides an ocean of liquid water beneath its surface. Because of the bright purity of Saturn’s rings – undarkened by interplanetary dust – astronomers have concluded that they must be relatively young. How they originated is not known, though that does not mean that they were created. A collision between two icy moons orbiting the planet is one possibility. Jupiter, Uranus and Neptune also have rings: faint, unspectacular ones that evoke no wonder.
Titan, the second largest moon in the solar system after Ganymede, has a water-ice crust with lakes and rivers of liquid methane and ethane (C2H6). A saltwater ocean lies beneath its surface, and beneath that probably a mantle of frozen water.
Uranus is thought to be ‘water-rich’ in composition, but quantitative estimates vary, depending on the model (Helled et al. 2020); the atmosphere could contain a lot of water (H2O) and the interior relatively little. Uranus’s moons are predominantly rocky, but the four largest may have oceans beneath their icy crusts, with antifreeze substances such as ammonia and salts helping to keep the water liquid.
Neptune is named after the Roman god of the Ocean and its moons named after minor water deities. However, its blue colour is due to the absorption of red and infrared light by methane in the atmosphere. Little is known about its internal composition and still less about the moons’, except for Triton, the largest moon. Its surface is covered by frozen nitrogen, methane, carbon dioxide and water, and its interior thought to be two-thirds rock and one third ice, mainly water.
- the fragmentary nature of the TNOs – it is estimated to contain more than 100,000 objects over 50 km in size and, wildly contrary to computer models, quadrillions of objects 10–100 metres in size (Cooray 2006);
- the low total mass of the Kuiper Belt objects (more easily determined than that of the Scattered Disc);
- the ‘surprisingly high level of dynamical excitation’ of the objects – contrary to expectations, they have highly elliptical orbits at various angles to the ecliptic, not circular orbits all close to the plane (‘disc’ is a misnomer);
- the existence of more such bodies, known as the “scattered disc”, that extend in similarly erratic orbits beyond the Kuiper Belt and are essentially a continuation of it;
- the largest objects appear to be rocky.
The largest Kuiper Belt Objects (KBOs) are Pluto, Makemake, Eris, Haumea and Quaoar, all mainly rocky and classified as dwarf planets, but with a substantial ice component. Triton is larger than Pluto but of similar composition and almost certainly originated from the belt. Some of the smaller KBOs may be mainly ice. As with the asteroid belt, the vast number of these objects is thought to reflect the outcome of collisions between larger bodies. Thus the present state of the Kuiper Belt does not reflect its primeval state, and its more recent history may be one of disaggregation rather than aggregation.
As with the Sun, the composition of the KBOs has to be inferred from spectra of their surface. This is not straightforward, since interaction with polymer-producing cosmic rays and the Sun’s ultraviolet radiation has complicated the chemistry. Pluto, about one third the size of Earth’s moon by volume, is one third water and two-thirds rock. The ‘dynamical excitation’ of the objects formerly in its vicinity is evident from the thousands of impact craters dotting its surface.
In simple terms, the surfaces of the largest bodies are mainly nitrogen and methane whereas the surfaces of the small to medium-sized objects are mainly water-ice. Most of the smaller bodies are fragments of larger ones and therefore younger. Some of the water ice is crystalline and must have formed in temperatures well above those now prevailing. This may not have been long ago, since cosmic rays will reduce crystalline ice to an amorphous state within 0.1–1.0 million years.
In view of the problems associated with the nebula hypothesis, one should ask whether a creation-based approach might not offer a better interpretation. Many pre-scientific peoples had a tradition that a celestial ocean existed above the terrestrial one. The Egyptians visualised the Sun as travelling through the sky in a boat. Babylon’s creation myth, Enuma Elish, related that Tiamat, a personification of the primordial waters, was split into an upper and a lower ocean. According to the Hebrew tradition, God made the firmament of the solar system by separating the waters into two, one region above the firmament and the other below.
Ancient tradition and modern astronomical knowledge are best integrated by supposing that the waters formed a protective, nebulous, circumambient shell not unlike the shape postulated for the Oort cloud. Because of the radiation emanating from the Milky Way’s once luminous nucleus, interstellar space was initially much warmer than today. Over time, much of the vapour diffused inwards under the influence of the Sun’s gravity. By 4.567 Ga ago in geological time, interplanetary space may have hosted a substantial volume of water vapour. Water requires a non-gaseous surface known as a ‘cloud condensation nucleus’ to make the transition from vapour to liquid, and the most plentiful source of such nuclei would have been dust particles from the explosion of the largest rocky planets and, later, from the surfaces of the remaining rocky planets when they were struck by the debris – hence the large volume of water that showered down on Mars in its Noachian period. Beyond Neptune, cooling and electrostatic sticking would have caused the droplets to consolidate into ice particles. Both the Kuiper Belt and the Scattered Disc can be understood as the mixed remains of water from the circumambient shell and rock from the outermost planets.
Consistent with the tradition that interplanetary space was created by dividing a once single body of water, isotopic ratios suggest that the Earth’s water and that locked up in comets had a common origin (Lis et al. 2019).
According to Genesis, the original Earth was destroyed in a terrible watery cataclysm. There were two agents of destruction. One was the springs of the great subterranean deep, which suddenly exploded and flooded the planet. The other was the rain that for 40 days fell through ‘apertures of the heavens’. As a consequence of the rain, all life was blotted out.
The rain is problematic, for it is difficult to model an atmosphere that might have held that amount of water, and also difficult to see how 40 days of rain, however torrential, could have drowned all the mountains and resulted in life’s total obliteration. Genesis implies that up to that point rain was unknown: the Earth’s surface was watered from below, not above. While some water must have evaporated from lakes and inland seas, and from the ground itself, this does not seem to have been a major component of the hydrological cycle. Possibly some water may have reached the Earth from interplanetary space and lingered in the upper atmosphere, but if so, what triggered its collapse?
On the other hand, water might not have been what primarily rained on the planet. In the story about Sodom and Gomorrah the text says that God rained sulphur and fire on the cities; on another occasion he rained down coals (Ps 11); in the book of Joshua he brings down on Israel’s enemies a hail of ‘large stones’, i.e. meteorites. An alternative, then, might be to understand the rain as an onslaught of asteroids, accompanied by water as impact-generated caused vapour in the atmosphere to condense and precipitate. Just such a bombardment is known to have pummeled the Earth early in its history, as attested by the giant impact craters on the Moon, just a stone’s throw away in astronomical terms. Asteroids would certainly have obliterated life on Earth.